Accuracy of rotor position detection relating to the control of brushless DC motors

Abstract

During operation of a 3 phase BLDC motor it is driven by use of a PWM waveform applied to one of the driven phase (curve a). The other driven phase is connected thereto but no driving signal is applied (curve b). The third phase is left floating (curve c). This allows the back EMF in the third phase to be monitored for the purpose of determining rotor position by detection of zero crossing points. The rapid switching of the PWM pulses causes ringing in the back EMF signal indicated for one pulse by the ringed portions 1 of curve c. The ringing in the back EMF signal introduces inaccuracy into position calculations derived from back EMF signal measurement. In order to reduce this ringing, in the present invention, a reverse pulse is applied to the other driving coil shown (curve b) prior to a PWM on pulse. The reverse pulse has a polarity such that it drives the phase current through the linked coils in a direction opposite to that caused by the PWM on pulse. This reverse pulse reduces the magnitude of the ringing in the back EMF signal.

The invention claimed is: 
     
       1. A method of monitoring the back EMF in a floating phase of a BLDC motor, the other phases being driven by a PWM waveform comprising a series of on pulses of a first polarity interspersed with zero signal periods, the method comprising:
 applying a reverse pulse prior to a scheduled PWM on pulse to the complementary stator phase to which the on pulse is applied, the reverse pulse having a duration of less than the subsequent PWM on pulse and having a polarity so that it drives the motor in a direction opposite to the direction in which the motor is driven by the on pulse; and 
 monitoring the back EMF in the floating phase during the final portion of the PWM on pulse. 
 
     
     
       2. A method as claimed in  claim 1  wherein the on pulse subsequent to a reverse pulse is of extended duration. 
     
     
       3. A method as claimed in  claim 1  wherein the method is applied at all PWM duty cycles. 
     
     
       4. A method as claimed in  claim 1  wherein the method is only applied during selected PWM duty cycles. 
     
     
       5. A method as claimed in  claim 1  wherein the reverse pulse is applied before each on pulse. 
     
     
       6. A method as claimed in  claim 1  wherein the reverse pulse is applied only before selected on pulses. 
     
     
       7. A method as claimed in  claim 1  wherein back EMF monitoring is carried out using a comparator or an analogue to digital converter to detect zero crossings. 
     
     
       8. A method as claimed in  claim 1  wherein the back EMF signal is low pass filtered or integration filtered before monitoring. 
     
     
       9. A brushless DC motor operable in accordance with the method of  claim 1 . 
     
     
       10. A method as claimed in  claim 2  wherein the on pulse is extended by a period equal to the duration of the reverse polarity pulse. 
     
     
       11. A method as claimed in  claim 2  wherein back EMF monitoring is carried out during the extended portion of an extended duration on pulse. 
     
     
       12. A method as claimed in  claim 2  wherein the on pulse has a duration determined by the ringing settling time. 
     
     
       13. A method as claimed in  claim 4  wherein the method is applied at PWM duty cycles where the duration of the on pulse is less than the ringing settling time. 
     
     
       14. A method as claimed in  claim 6  wherein the reverse pulse is applied intermittently. 
     
     
       15. A method as claimed in  claim 6  wherein the reverse pulse is applied before on pulses only when a zero crossing of the back EMF is expected or on opposite sides of an expected zero crossing. 
     
     
       16. A brushless DC motor as claimed in  claim 9  wherein the motor is a three phase BLDC motor.